gajim: Clean up and rewrite package expression.

This should clean up the package expression significantly by actually using
explicit input attributes and setting PYTHONPATH and GST_PLUGIN_PATH from
environment variables in the builder.

In addition, this adds a small patch from the upstream Mercurial repository to
add an index.theme to the icons, so Gajim is able to load them correctly from
the store.

With this change, I'm adding myself to the maintainers list as well, because I'm
switching over from TKabber to Gajim.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>